Well, for 150 bucks, a 7770 is a good bet.http://www.newegg.ca/Product/ProductList.aspx?Submit=ENE&DEPA=0&Order=BESTMATCH&N=-1&isNodeId=1&Description=7770&x=0&y=0
Take your pick.
You can also go with a HD6850, bit faster than the 7770:http://www.newegg.ca/Product/ProductList.aspx?Submit=ENE&DEPA=0&Order=BESTMATCH&N=-1&isNodeId=1&Description=6850&x=0&y=0
The 7770 is below your budget and the 6850 is just above (before sales), so look around.
Oh and if you want an Nvidia, try the 560 Ti or non-Ti editions. If you can get a Ti on sale, that's a good bet, probably the fastest performance wise, make sure your case has plenty of cooling/ventilation, your PSU will run the 560's fine, but I'm not sure about your case cooling. The 7770 and 6850's generate a lot less heat, so those are my first suggestions on a brand name box (since cooling is always limited on these boxes).